1. Field of the Invention
This invention relates to an air-fuel ratio control system for controlling the air-fuel ratio of a fuel mixture for an internal combustion engine.
2. Description of the Related Art
The air-fuel ratio A/F of a fuel mixture delivered into an engine is basically established according to the amount of intake air, which depends upon a throttle valve opening (TVO) of a throttle valve operated by the driver, and the speed of rotation of the engine. This basic air-fuel ratio is also correctly controlled to make a fuel mixture richer or leaner according to engine operating conditions so as to be most suitable for both of actual engine operating conditions and vehicle driving characteristics. Incidentally, in order to meet strict emission regulations, vehicles have been equipped with exhaust gas purifying devices which make the use of three-way catalytic converters. Such a three-way catalytic converter is able to perform both oxidation
carbon monoxide (CO) and hydrocarbons (HC) and deoxidation or reduction of nitrogen oxides (NOx) coincidentally only within a considerably narrow range (window) of air-fuel ratio in the vicinity of a theoretically ideal air-fuel ratio (A/F=14.7 or =1), and purify the exhaust gas by changing it into carbon dioxide (CO.sub.2), water (H.sub.2 O), oxygen (O.sub.2), and nitrogen (N.sub.2), which do not pose a health problem to human body. In other words, the exhaust gas purifying devices with the use of three-way catalytic converters discharge nitrogen dioxides (NOx) if an air-fuel ratio is shifted on a lean side from the theoretically ideal air-fuel ratio of 14.7 or .lambda.=1, and carbon monoxide (CO) and hydrocarbons (HC), if shifted to a rich side from the theoretically ideal air-fuel ratio of 14.7 or .lambda.=1.
Accordingly, in order to realize effective activities of the three-way catalytic converter so as to assure adequate and sufficient purification of exhaust gas, it is necessary for the engine to maintain the actual air-fuel ratio of a fuel mixture at the theoretically ideal air-fuel ratio of 14.7 according to operational requirements of the engine with accuracy and reliability as high as possible.
However, because the allowable range (window) of the theoretically ideal air-fuel ratio, which is defined as .lambda.=1.+-.a, is too narrow to purify carbon monoxide (CO) and hydrocarbons (HC) and nitrogen dioxides (NOx) all at once, it is not feasible to meet the rigorous emission regulations as described above through an ordinary open loop control of air-fuel ratio. For this reason, conventionally, an electronic feedback or closed loop control of air fuel ratio has been conducted to control the amount of fuel delivered into the engine according to changes in actual air-fuel ratio as soon as possible so as to develop reliably and maintain the actual air-fuel ratio within the allowable range (window) of the theoretically ideal air-fuel ratio (14.7.+-..phi. or .lambda.=1.+-.a) as a target air-fuel ratio. In this electronic air-fuel ratio feedback control, changes in actual air-fuel ratio are equivalently determined based on the concentration of oxygen in the exhaust gas, which is detected as an emission air-fuel ratio by an oxygen (O.sub.2) sensor with a high accuracy. The exercise of such an electronic feedback control realizes the sufficiently satisfied purification of exhaust gas.
Three-way catalytic converters, which have their own active temperatures, exhibit adequate purification characteristics only for the temperatures of exhaust gases above a critical level. Consequently, within a certain period of time after starting of an engine, it is general to suspend the feedback of the concentration of oxygen (O.sub.2) and then conduct an open loop control so as to enrich a fuel mixture, thereby providing an air-fuel ratio higher than the theoretically ideal air-fuel ratio in order to accelerate warming-up of the engine. Once the engine has been warmed up, the oxygen (O.sub.2) concentration related feedback control is resumed. However, it takes a long period of time until the catalytic converter develops its efficient performance, and during the long period of time, emissions are extremely poor.
In recent years, air-fuel ratio control systems have been proposed, which enables a three-way catalytic converter to develop its efficient performance of exhaust gas purification even during engine warming-up as a result of coercively causing alternative deflections of an air-fuel ratio toward richer and leaner sides by a specified rate during the oxygen (O.sub.2) concentration related feedback control, as shown in FIG. 1. Such an air fuel ratio control system is known from, for instance, Japanese Unexamined Patent Publication No. 2-230935.
In other words, it has been proved that, even before catalytic converters have become sufficiently activated, when the air-fuel ratio control is made to cause large coercive deflections in exhaust gas air-fuel ratio from the theoretically ideal air-fuel ratio of =1, the catalytic converters are enabled to purify a part of exhaust gas. This is because, when the emission air-fuel ratio is changed toward the richer side, oxygen (O.sub.2) completely disappears from the surface of catalyst and reduction or deoxidation is significantly provided and, conversely, when it is changed toward the leaner side, chemical reaction is accompanied by accelerated oxidation thanks to the presence of a sufficient quantity of oxygen (O.sub.2), resulting in improved purification of pollutants, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x).
However, with this air-fuel ratio control system described in the above-mentioned publication, as the temperature of exhaust gas rises with the progress of engine warming-up, it is rendered difficult to restore an air-fuel ratio into the allowed range (window) of the theoretical air-fuel ratio of .lambda.=1.+-.a due to large coercive deflections.